1. Field of the Invention
The instant invention is directed toward endless fabrics, and more particularly, fabrics used as multi-layer woven creping fabrics in the production of paper products. More particularly, the instant invention is directed to creping fabrics used in the production of products such as paper and sanitary tissue and towel products.
2. Description of the Prior Art
Soft, absorbent disposable paper products, such as facial tissue, bath tissue and paper toweling, are a pervasive feature of contemporary life in modern industrialized societies. While there are numerous methods for manufacturing such products, in general terms, their manufacture begins with the formation of a cellulosic fibrous web in the forming section of a papermaking machine. The cellulosic fibrous web is formed by depositing a fibrous slurry, that is, an aqueous dispersion of cellulose fibers, onto a moving forming fabric in the forming section of a papermaking machine. A large amount of water is drained from the slurry through the forming fabric, leaving the cellulosic fibrous web on the surface of the forming fabric.
Further processing and drying of the cellulosic fibrous web generally proceeds using one of two well-known methods. These methods are commonly referred to as wet-pressing and throughdrying. In wet pressing, the newly formed cellulosic fibrous web is transferred to a press fabric and proceeds from the forming section to a press section that includes at least one press nip. The cellulosic fibrous web passes through the press nip(s) supported by the press fabric, or, as is often the case, between two such press fabrics. In the press nip(s), the cellulosic fibrous web is subjected to compressive forces which squeeze water therefrom. The water is accepted by the press fabric or fabrics and, ideally, does not return to the fibrous web or paper.
After pressing, the paper is transferred, by way of, for example, a press fabric, to a rotating Yankee dryer cylinder that is heated, thereby causing the paper to substantially dry on the cylinder surface. The moisture within the web as it is laid on the Yankee dryer cylinder surface causes the web to adhere to the surface, and, in the production of tissue and toweling type paper products, the web is typically creped from the dryer surface with a creping blade. The creped web can be further processed by, for example, passing through a calender and wound up prior to further converting operations. The action of the creping blade on the paper is known to cause a portion of the interfiber bonds within the paper to be broken up by the mechanical smashing action of the blade against the web as it is being driven into the blade. However, fairly strong interfiber bonds are formed between the cellulosic fibers during the drying of the moisture from the web. The strength of these bonds is such that, even after conventional creping, the web retains a perceived feeling of hardness, a fairly high density, and low bulk and water absorbency.
In order to reduce the strength of the interfiber bonds that are formed by the wet-pressing method, throughdrying can be used. In the throughdrying process, the newly formed cellulosic fibrous web is transferred to a through-air-drying (TAD) fabric by means of an air flow, brought about by vacuum or suction, which deflects the web and forces it to conform, at least in part, to the topography of the TAD fabric. Downstream from the transfer point, the web, carried on the TAD fabric, passes through and around through-air-dryer, where a flow of heated air, directed against the web and through the TAD fabric, dries the web to a desired degree. Finally, downstream from the through-air-dryer, the web may be transferred to the surface of a Yankee dryer for further and complete drying. The fully dried web is then removed from the surface of the Yankee dryer with a doctor blade, which foreshortens or crepes the web thereby further increasing its bulk. The foreshortened web is then wound onto rolls for subsequent processing, including packaging into a form suitable for shipment to and purchase by consumers.
In the TAD process, the lack of web compaction, such as would occur in the wet-pressing process when the web is pressed in a nip while on the fabric and against the Yankee drying cylinder when it is transferred thereto, reduces the opportunity for strong interfiber bonds to form, and results in the finished tissue or towel product to have greater bulk than can be achieved by conventional wet-pressing. Generally, however, the tensile strength of webs formed in the through-air drying process is not adequate for a finished consumer product, and various types of chemical additives are typically introduced into the web prior to and/or during the forming operation to achieve the desired strength while still retaining most of the bulk of the original product.
As noted above, there are many methods for manufacturing bulk tissue products, and the foregoing description should be understood to be an outline of the general steps shared by some of the methods. For example, the use of a Yankee dryer is not always required, as, in a given situation, foreshortening may not be desired, or other means, such as “wet creping”, may have already been taken to foreshorten the web.
Other process and machine configuration variations of either wet pressing or through-air-drying are also to be considered here. For example, in some cases, no creping doctor is employed when the sheet is removed from the dryer surface. Further, there are processes that are alternatives to the through-air-drying process that attempt to achieve “TAD-like” tissue or towel product properties without the TAD units and high energy costs associated with the TAD process.
The properties of bulk, absorbency, strength, softness, and aesthetic appearance are important for many products when used for their intended purpose, particularly when the fibrous cellulosic products are facial or toilet tissue or paper towels. To produce a paper product having these characteristics, a fabric will often be constructed such that the sheet contact surface exhibits topographical variations. These topographical variations are often measured as plane differences between woven yarn strands in the surface of the fabric. For example, a plane difference is typically measured as the difference in height between a raised weft or warp yarn strand or as the difference in height between machine-direction (MD) knuckles and cross-machine direction (CD) knuckles in the plane of the fabric's surface. Often, the fabric structure will exhibit pockets in which case plane differences may be measured as a pocket depth.
It should be appreciated that these creping fabrics may take the form of endless loops on the paper machine and function in the manner of conveyors. It should further be appreciated that paper manufacture is a continuous process which proceeds at considerable speeds. That is to say, the fibrous slurry is continuously deposited onto the forming fabric in the forming section, while a newly manufactured paper sheet is continuously wound onto rolls after it is dried.
The instant invention provides a fabric that may reduce or even prevent rewetting of a product being formed thereon during such operation.